Phonon Echo and Ionic Diffusion in Powdered Superionic Conductor

Abstract

Lithium transition metal oxides are attractive as the positive electrode of secondary battery. Li⁺ ionic diffusion plays an important role in the electrode materials. In this study, the internal friction was measured by the phonon echo technique to study the Li⁺ ionic motion in polycrystalline LiNbO₃ powders. The Phonon echo is generated through the non-linear interaction between acoustic waves and lattice vibration. The inverse decay time of the echo, 1/T₂ is connected with a magnitude of the internal friction of each particle, which is functional as an acoustic oscillator. The samples were ground and were arranged to be the size of 46 - 53μm in diameter with stainless steel mesh. Phonon echo measurements were carried out at 40 MHz in the temperature range of 300K to 900K. The decay time T₂ showed the significant decrease above 800K. The temperature dependence of T₂ was explained in terms of a Debye-type relaxation model based on the Li⁺ ionic motion. It yields the activation energy of about 0.93 eV. This result is in fairly good agreement with those of NMR and ionic conductivity measurements. Further investigation is now in progress, but the present study seems to prove that the phonon echo measurement is useful to elucidate the ionic motion in ionic conductors.